The present invention relates to a semiconductor device and more particularly to a semiconductor device that includes an edge passivation structure.
A typical semiconductor device usually includes an active region which terminates at a PN junction.
The breakdown down voltage of a PN junction formed in a substrate is usually less than its theoretical limit because certain locations on the PN junction have a tendency to develop higher electric fields. The PN junction at the terminal edge of the active region of a device, for example, experiences higher electric fields particularly at locations of low radius of curvature.
To reduce the intensity of the electric fields near the PN junction at the terminal edge of the active region, high voltage semiconductor devices may include an edge passivation structure. An edge passivation structure provides a transition region in which the high electric fields around the active area change gradually to the lower potential at the edge of the device. A passivation structure lowers the field intensity around the termination region of a PN junction by spreading the electric field lines across the edge passivation region.
U.S. Pat. No. 5,382,825 discusses a variety of passivation structures and their respective drawbacks. To improve on the prior art passivation structures discussed therein, U.S. Pat. No. 5,382,825 discloses a passivation structure which includes a spiral ribbon of resistive material disposed around the active region of a semiconductor device to gradually relieve the electric fields near the termination region of a PN junction in a semiconductor device.
The spiral ribbon disclosed by U.S. Pat. No. 5,382,825 never crosses itself. As a result, the position of the end of the spiral ribbon closest to the active region determines the initial voltage from which voltage drop around the active region begins. This initial voltage, however, may not be representative of the intensity of the electric field around the active region.
To “equalize” the voltage, U.S. Pat. No. 5,382,825 suggests forming a field plate around the active region. The suggested width for the field plate is three times the depth of the PN junction surrounding the active region. This width could occupy a large area on the surface of the die, thereby increasing the size of the die.
Also, the spiral ribbon has a width which varies so that the resistance of the ribbon is equalized throughout its length. In practice, such a ribbon may be difficult to manufacture with precision. As a result the resistance along the length of the ribbon may vary in an unpredictable manner leading to fluctuations in the voltage drop rather than a smooth, near linear voltage drop.